Electromagnetic reproduction of sound and other wave motions



March s, 1938. J F.M, EST 2,110,193

ELECTROMAGNETIC REPRODUCTION OF SOUND AND O THER WAVE MOTIONS 1 Filed June 9', 192a Patented Mar. 8 1938 PATENT OFFICE ELECTROMAGNETIC REPRODUCTION OF solmn OTHER WAVE MOTIONS Frank M. Best, New York, N. Y., assignor to Dorothy Sparrow Best Application June 9, 1928, Serial No. 284,020

4 Claims.

n invention relates a new and improved method for the electromagnetic reproduction of sound and other wave motions, and a new and I {improved mechanism therefor.

.45" One of the'objects of'iny invention isto provide a new and improvedj method whereby the audiojcurrents produced bya radio receivingset, electric pick-up, or the like,lcan be' caused to reproduce the original sound waves, with great 10 volume, and" also with substantially uniform fidelity over a. very wide range of tone.

Another object of my invention is to provide "anf'improved driving unit for telepho'nes,loud speakers or the like, which can deliver sound of 15 great volumeand'of correct tone. Another object of my invention is to provide a driving unit in which sound waves, or other similar waves having frequencies above the audiblerange, shall be reproducedby means of waves zo electromagnetically inducedin a suitable armature, and with little or' no bodily movement of the armature itself.

The present invention ,also contemplates an electromagnetic device for translating. electrical 25 vibrations into mechanical vibrations wherein V this translation "15 accomplished vby, vibratory v movements within an armature supported in sorigid a manner tobeimmovable. It has a stationary magnetiza'ble field. structure provided with 30 fixed field coils'to receive the voice currents (or other current's of an'inconstant nature), and an armature rigidly connected to a resonator such as'a sounding board, anda rigid mounting for the armature.

3 i For sound reproduction" the preferably form of resonator is made of light wood such as spruce impregnated with rubber solution or natural latex, the rubber being inthe unvulcanized state. These sounding boards may range in size. from 40 20}: 30in. to 20 x 32 ft. ,When so operated these a sounding boards (made for exampleasshown in my application Serial No. 458,598, filed May 31, 1930, Methods of, and. apparatus for utilizing energy of a vibratory nature) are capable of re- 45 producing sounds throughoutthe entire audible range from 13 vibrations to 17,000 vibrations per "second and indeed go far above the limits of audibility, forexample to at least 40,000 vibrations per second. 'Thesesounding boards may be 50 from one-half to, one-quarter of an inch thick and are secured to exceedingly rigid supports which in-turn carry the field structure 'of the electromagnetic units so that there is no possibility of movement of the unit relative to the resonator.

55 One or more units may be employed and the (Cl'. 179-110) V armature of each unit is rigidly connected to the sounding board. Each specific area of the resonator has a natural frequency and these natural frequencies extend throughout the range above given. "When the coils of the units are 5 energized, the particular area of the sound board or resonator having the natural frequencies of the energizing current receives, this vibratory energyand acts as a reflex driver to multiplythe efiect many times. The resonator is so rigid and strong that the unit does not have thepower that would be required for bending or deflecting it (as a beam) but yet it may be set into violent vibration by a very small input in the voice coils. With other forms of resonator or driver the armature 15 may be made to vibrate at frequencies in the order of several hundred thousand gycles per second.

The vibratory movementof' the armature appears to be within the armature itself and not to be in the nature of a bodily movement of the armature. It seems to be analogous to the vibration of the armature of an ordinary electric motor which has been overloaded until it stalls, whereupon it starts to vibrate. Very narrow clearances may be employed between the armature and field structure without striking of the parts and careful tests negative thepossibility of actual; bodily movement of the armature. v V

" Other objects of my invention will be set forth in the following description and drawing which illustrates a preferred embodiment thereof, it being understood that the above general state- Iment of the objects of, my invention is intended merely to generallyexplain the same and-not to limit it in any manner.

, H Fig. 1 is a top view illustrating one'emb'odiment of. my invention.

' Fig. 5 is'a diagrammatic view of one of the circuits that may be employed. I v

In making driving units for loud speakers or the like, it has heretofore been customary to use an armature resiliently supported. between the poles of a magnet. which was either a permanent magnet or else had fieldcoils. The field of the magnet was varied by means of current passing through a coil which was either wound upon the pole pieces of the magnet or else upon the armature itself. This current was for example, the audio current output of a radio receiving set so that the armature was caused to vibrate in accordance with the audio currentvariations, to

actuate a diaphragm which was either the diaphragm of a horn, or else vibrated in free air.

These resiliently supported armatures of the prior art had bodily movement as well as vibratory movement and, hence, the inertia of the armature always seriously affected the correctness of the reproduced sound and in addition such unit could not take much powenbecause, it thearmature vibrated too far it collided against the adjacent pole pieces due to the fact that ,a' small air gap in the magnetic circuit was necessary for maximum efliciency. i

According to my invention the armature is rigidly mounted so that if there is any movement thereof as a whole, it' is imperceptible. The varying electromagnetic field produced by the audio current is utilized for producing vibratory waves in the material or the armature itself unaccompanied by bodily movement. These waves may be compared with the waves in a sonorous body such as agbell', a length of railroad tr I f across the adjacent air gaps. I prefer .to so wind or'the like; when tapped by a light blow. Such a blow produces a large volume of sound without bodily movement. The wavesiproduced in the armature are (taken up by the rigid support and "reflexed or reflected baclr'and forth by it so as to build 111 5 *the amplitude-of the vib ation without bodily movement. 'These' amplified waves are transmittedto the diaphragmor other'mechanical element which produces sound waves. I

As' shownin Fig.1, one of the embodiments "jofmy device comprises a magnet M 'whichmay eitherbe a, permanent magnet orfmay' havea field coil. ,I have found that satisfactory results 1 can be secured witna; permanent magnet satugrated'so as, to have a maximum pull. The mag- I net is provided with polepieces l and 2, which are provided with inwardly directedprojections 3. l. 5 andirfFour'audio or actuating cells C are wound 'uponthe extensionsl, l, 5 and, the

said coils beingarranged in pairs, each pair being located upon apair of adjacent pole piece extensions;

A's shown'in Fig.4, the ar at re A has en.

- other permeable" ma I residualmagnetism.

armature A may bemade of soft".iron, or any terial I having'f little for no A yoke Y has" its endsconnected to the pole pieces and to their extensions by meansoiibolts I and and nuts 9 and 10.: The poleipieces I 1 bymeans of screws l9 and 20. Theiblock B is connected to yoke'Y'by screw H. 'A terminal member II is clamped between nut l0 and an upper nut l0. 'I'heterminal member H' is made of insulating material andtwo binding posts I2 having terminal lugs l6 are mounted thereon.

Theleads L for the, coils Care connected to'said "binding posts. The coils C'may be connected in any suitable manner, to vary the impedance of the circuit as desired. In the embodiment shown, the four coils C are shown as connected in seriesparallel, to provide a circuit 01 maximum impedancepbut thi's'is merely illustrative. i

the rubber being" in the unvulcanizedstate. I alsomay use several units with a single sound ing board capable of vibration over a wide range, each unit being intended to reproduce a given range of tones, as determined by the characteristics of the immediate neighborhood of the sounding board. The units for reproducing the higher tones are at or near the centre of the 4 flat sounding board. The block B is rigidly conv nected by screw I to the middle of yoke Y.

The operation of the device is as follows: Assuming that the pole-piece projections 8 and 5 are North and the pole-piece projections l and 6 are South, the magnet flux passes alongthe coils as to simultaneously and equally vary the magnetic'flux at eaenenam the armatureA- However, it would not be departing from theinvention if the flux was strengthenedfat one end -otthe armature A and was simultaneously weakened at the other end thereof. 7

sincea very strong magnetic field is produced by the magnet M, and the air gaps are very narrow, the varying magnetic field produces waves in the'material of armature Aj The nature of thesewaves is not definitely understood but from certain experiments they seem to be 01' a rotary.

or oscillatory nature about, the, axis 01 the supporting bar somewhat analogous to the rotation of the vibratory waves, in ,a' bell when it is sounded. The waves produced in the armature are transmitted into the'rigid support which returns them toward the armature. This process is continued so that thema'gnitude-otthewaves is built up. Their magnitude is exceedingly small even though the sounding boardtowhich the armature is attached may have a ripple-like vibration of considerable magnitude and easily felt by the hand,'with'out any defleotionoi the sounding board. These waves 'arertransmitted along'themetal rod IE to the diaphragm, sounding board or the like. 'These vibrations are reflexed'in the material of the armature and the diaphragm and propagate'sound of great penetration and volume and with remarkablefldelity. Hence considerable power can bej'supplied tothe coils C without causing the armature A to click against the 'pole piece, andsunicient power should be supplied to produce waves in the material of the armature A. It is clear that all the parts, save the magnet M, form a connected unit, which can be readily attachedto the ends of the magnet M. It deabutting the lateral faces of the ends of thelegs 'of said magnet.

I prefer'thatthe block B, andthe armature supporting arm A? should not be integral with armature so that said arm A can bemade of metal having greater strength than soft iron.

"The arm A" and block B could be made of brass, bell metal or other non-permeable material. Likewise, the yoke Y is made of non-permeable material. I I

' Owingtothe rigid mounting of the armature and the absence of bodily movement, inertia eflects are eliminated and wave motions far above the frequencies of sound are available. The magnetic structure above shown is useful in other fields than sound reproduction, such for example as so called high frequency generation, when several of these units are combined, some used as motors and others as generators, all directly connected.

I have shown a preferred embodiment of my invention, but it is clear that numerous changes and omissions could be made without departing from its spirit.

I claim:

1. A loud speaker unit having two pairs of pole members, an armature made of permeable material and located between the pole members, said armature having a rigid support so as to be bodily immovable, so that four air gaps of substantially constant width are formed in the magnetic circuit, and coil means associated with each pole member and adapted to vary the flux through said air gaps and set up vibration within the armature and its support without producing bodily movement of either the armature or its support.

2. A loud speaker unit having two pairs of pole members, an armature made of permeable material and located between the pole members, said armature having a rigid support so as to be bodily immovable, so that four air gaps oi. substantially constant width are formed in the magnetic circuit, and coil means associated with each pole member and adapted to vary the flux through said air gaps and set up vibration within the armature and its support without producing bodily movement of either the armature or its support, said coil means comprising four coils respectively located on said pole members.

3. A loud speaker driving unit comprising a magnet, and a sub-unit connected to the pole portions of said magnet, said sub-unit comprising a yoke, each end of said yoke having a pole-piece and two pole-piece projections connected thereto, each pole piece being intermediate two polepiece projections, a supporting block connected to said yoke intermediate the ends thereof, said block having an armature rigidly connected thereto, said armature being located intermediate the adjacent ends of said pole-piece projections to form four air gaps in the magnetic circuit, coils respectively wound on said pole-piece projections, said sub-unit being directly connected to said end-portions of the magnet through said pole-pieces.

4. A loud speaker operating unit, comprising a support, a stationary magnet carrying a plurality of pairs of rigid, immovable pole pieces, energizing coils wound about the pole pieces and arrangedto produce opposite poles adjacent an armature, an armature having tips close to the pairs of pole pieces and mounted on the support, the support being substantially rigid when subjected to attractive forces within the power of the magnet, and a connection from the armai ture for transmitting sound wave vibrations therein to a sonorous body.

- FRANK M. BEST. 

